by Renu Tewari

Posted in Technical | May 13, 2019 6 min read

Apache Kafka has become an essential component of enterprise data pipelines and is used for tracking clickstream event data, collecting logs, gathering metrics, and being the enterprise data bus in a microservices based architectures. Kafka is essentially a highly available and highly scalable distributed log of all the messages flowing in an enterprise data pipeline. Kafka supports internal replication to support data availability within a cluster. However, enterprises require that the data availability and durability guarantees span entire cluster and site failures.

The solution, thus far, in the Apache Kafka community was to use MirrorMaker, an external utility, that helped replicate the data between two Kafka clusters within or across data centers. MirrorMaker is essentially a Kafka high-level consumer and producer pair, efficiently moving data from the source cluster to the destination cluster and not offering much else. The initial use case that MirrorMaker was designed for was to move data from clusters to an aggregate cluster within a data center or to another data center to feed batch or streaming analytics pipelines. Enterprises have a much broader set of  use cases and requirements on replication guarantees.

Multiple vendors and Internet service companies have their own proprietary solutions (Brooklin MirrorMaker from Linkedin, Mirus from Salesforce, uReplicator from Uber, Confluent Replicator from Confluent) for cross-cluster replication that points to the need for the community Apache Kafka to have an enterprise ready cross-cluster replication solution too.

Typical MirrorMaker Use Cases

There are many uses cases why data in one Kafka cluster needs to be replicated to another cluster. Some of the common ones are:

Aggregation for Analytics

A common use case is to aggregate data from multiple streaming pipelines possibly across multiple data centers to run batch analytics jobs that provide a holistic view across the enterprise, for example, a completeness check that all customer requests had been processed..

Data Deployment after Analytics

This is the opposite of the aggregation use case in which the data generated by the analytics application  in one cluster (say the aggregate cluster) is broadcast  to multiple clusters possibly across data centers for end user consumption.

Isolation

Sometimes access to data in a production environment is restricted for performance or security reasons and data is replicated between different environments to isolate access.  In many deployments the ingestion cluster is isolated from the consumption clusters.

Disaster Recovery

One of the most common enterprise use cases for cross-cluster replication is for guaranteeing business continuity in the presence of cluster or data center-wide outages. This would require application and the producers and consumers of the Kafka cluster to failover to the replica cluster.

Geo Proximity

In geographically distributed access patterns where low latency is required, replication is used to move data closer to the access location.

Cloud Migration

As more enterprises have an on prem and cloud presence Kafka replication can be used to migrate data to the public or private cloud and back.

Legal and Compliance

Much like the isolation uses case, a policy driven replication is used to limit what data is accessible in a cluster to meet legal and compliance requirements.

Limitations of MirrorMaker v1

MirrorMaker is widely deployed in production but has serious limitations for enterprises looking for a flexible, high performing and resilient mirroring pipeline. Here are some of the concerns:

Static Whitelists and Blacklists

To control what topics get replicated between the source and destination cluster MirrorMaker uses whitelists and blacklists with regular expressions  or explicit topic listings. But these are statically configured. Mostly when new topics are created that match the whitelist the new topic gets created at the destination and the replication happens automatically. However, when the whitelist itself has to be updated, it requires MirrorMaker instances to be bounced. Restarting MirrorMaker each time the list changes creates backlogs in the replication pipeline causing operational pain points.

No Syncing of Topic Properties

Using MMv1, a new or existing topic at the source cluster is automatically created at the destination cluster either directly by the Kafka broker, if auto.create.topics is enabled, or by MirrorMaker enhancements directly using the Kafka admin client API. The problem happens with the configuration of the topic at the destination. MMv1 does not promise the topic properties from the source will be maintained as it relies on the cluster defaults at the destination. Say a topic A had a partition count of 10 on the source cluster and the destination cluster default was 8, the topic A will get created on the destination with 8 partitions. If an application was relying on message ordering within a partition to be carried over after replication then all hell breaks loose. Similarly, the replication factor could be different on the destination cluster changing the availability guarantees of the replicated data. Even if the initial topic configuration was duplicated by an admin, any dynamic changes to the topic properties are not going to be automatically reflected. These differences become an operational nightmare very quickly.

Manual Topic Naming to avoid Cycles

By default, MirrorMaker creates a topic on the destination cluster with the same name as that on the source cluster. This works fine if the replication was a simple unidirectional pipeline between a source and destination cluster. A bidirectional active-active setup where all topics in cluster A are replicated to cluster B and vice versa would create an infinite loop which MirrorMaker cannot prevent without explicit naming conventions to break the cycle. Typically the cluster name is added in each topic name as a prefix with a blacklist to not replicate topics that had the same prefix as the destination cluster.  In large enterprises with multiple clusters in multiple data centers it is easy to create a loop in the pipeline if care is not taken to set the naming conventions correctly.

Scalability and Throughput Limitations due to Rebalances

Internally, MirrorMaker uses the high-level consumer to fetch data from the source cluster where the partitions are assigned to the consumers within a consumer group via a group coordinator (or earlier via Zookeeper). Each time there is a change in topics, say when a new topic is created or an old topic is deleted, or a partition count is changed, or when MirrorMaker itself is bounced for a software upgrade, it triggers a consumer rebalance which stalls the mirroring process and creates a backlog in the pipeline and increases the end to end latency observed by the downstream application. Such constant hiccups violate any latency driven SLAs that a service dependent on mirrored pipeline needs to offer.

Lack of Monitoring and Operational Support

MirrorMaker provides minimal monitoring and management functions to configure, launch and monitor the state of the pipeline and has no ability to trigger alerts when there is a problem. Most enterprises require more than just the basic scripts to start and stop a replication pipeline.

No Disaster Recovery Support

A common enterprise requirement is to maintain service availability in the event of a catastrophic failure such as the loss of the entire cluster or an entire data center. Ideally in such an event, the consumers and producers reading and writing to a cluster should seamlessly failover to the destination cluster and failback when the source cluster comes back up. MirrorMaker doesn’t support this seamless switch due to a fundamental limitation in offset management. The offsets of a topic in the source cluster and the offset of the replica topic can be completely different based on the point in the topic lifetime the replication began. Thus the committed offsets in the consumer offsets topic are tracking a completely different location at the source than at the destination. If the consumers make a switch to the destination cluster they cannot simply use the value of the last committed offset at the source to continue.  One approach to deal with this offset mismatch is to rely on timestamps (assuming time is relatively in sync across clusters). But timestamps get messy too and we will discuss that at length in the next blog in the series, “A look inside MirrorMaker 2.

Lack of Exactly Once Guarantees

MirrorMaker is not setup to utilize the support for exactly once processing semantics in Kafka and follows the default at least once semantics provided by Kafka. Thus  duplicate messages can show up in the replicated topic especially after failures, as the produce to the replicated topic at the destination cluster and the update to the __consumer_offsets topic at the source cluster  are not executed together in one transaction to get exactly once replication. Mostly it is a problem left to the downstream application to handle duplicates correctly.

Too many MirrorMaker Clusters

Traditionally a MirrorMaker cluster is paired with the destination cluster. Thus there is a mirroring cluster for each destination cluster following a remote-consume and  local-produce pattern. For example, for 2 data centers with 8 clusters each and 8 bidirectional replication pairs there are 16 MirrorMaker clusters.  For large data centers this can significantly increase the operational cost.  Ideally there should be one MirrorMaker cluster per destination data center. Thus in the above example there would be 2 MirrorMaker clusters, one in each data center.

What is coming in MirrorMaker 2

MirrorMaker 2 was designed to address the limitations of MirrorMaker 1 listed above. MM2 is based on the Kafka Connect framework and has the ability to dynamically change configurations, keep the topic properties in sync across clusters and improve performance significantly by reducing rebalances to a minimum. Moreover, handling active-active clusters and disaster recovery are use cases that MM2 supports out of the box. MM2 (KIP-382) is accepted as part of  Apache Kafka. If you’re interested in learning more, take a look at Ryanne Dolan’s talk at Kafka Summit, and standby for the next blog in this series for  “A Look inside MirrorMaker 2”.

Renu Tewari

Director Of Engineering

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